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28 Unit 1

The Physical World

Why It MattersUnderstanding that Earth is part of a larger physical system called the solar system helps us under-stand how life on our planet is possible. Earth’s physical sys-tems —air, landforms, and water — are affected by natural forces such as earthquakes and volcanoes that can influence human activities on the planet.

Section 1: Planet EarthPhysical processes shape Earth’s surface. As part of a larger system called the solar system, Earth has water, land, and air that make the planet suitable for plant and animal life.

Section 2: Forces of ChangePhysical processes shape Earth’s surface. The surface of the Earth has been shaped by the internal forces of tectonic activity, as well as by the external forces of weather-ing and erosion.

Section 3: Earth’s WaterPhysical processes shape Earth’s surface. The water cycle keeps the amount of water on Earth, both freshwater and salt water, fairly constant.

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Kilauea, in Hawaii, is one of the most active volcanoes in the world.

Jim Sugar/CORBIS

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Chapter 2 29

Organizing Information Make a Three-Pocket Book to help you orga-nize information about the physical systems and processes that affect life on Earth.

Reading and Writing As you read this chapter, write information on note cards about the structure of planet Earth and its place in the solar system, the natural forces that affect Earth’s physical systems, and Earth’s water. Place the cards in the correct pocket of your Foldable.

SECTION 1 Planet EarthAn astronaut, seeing Earth from the blackness of space, described it as “piercingly beautiful.” From the vantage point of space, the Earth’s great beauty resem-bles a blue and white marble, with contrasts of water and land beneath huge swirls of white clouds.

Voices Around the World“To the ancient Egyptians the heavens were almost close enough to touch—a benign canopy of light and dark held up by mountain peaks. But modern science has exploded that ancient, peaceful mirage, replacing it with . . . change, and processes that sometimes defy human understanding. Guided by leaps of imagination and armed with potent new technologies . . . scientists have . . . claimed the universe itself as a titanic laboratory.”

—Kathy Sawyer, “Unveiling the Universe,”

National Geographic, October 1999

Guide to ReadingSection PreviewEarth is part of a larger system called the solar system. Earth has water, land, and air that make it suitable for plant and animal life.

Content Vocabulary• hydrosphere (p. 32)• lithosphere (p. 32)• atmosphere (p. 32)• biosphere (p. 32)• continental shelf (p. 33)

Academic Vocabulary• approach (p. 31)• assistance (p. 32)• features (p. 33)

Places to Locate• Isthmus of Panama (p. 33)• Sinai Peninsula (p. 33)• Mount Everest (p. 33)• Dead Sea (p. 33)• Mariana Trench (p. 33)

Reading StrategyCategorizing As you read about Earth, complete a graphic organizer similar to the one below by describ-ing the four components of Earth.

Component Description

Hydrosphere

Lithosphere

Atmosphere

Biosphere

An astronomer with radio telescopes

30 Unit 1

Roger Ressmeyer/CORBIS

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Our Solar SystemMAIN MAIN IdeaIdea Earth is part of a larger physical sys-

tem that contains other planets, moons, and stars.

GEOGRAPHY AND YOU Have you ever seen a movie or read a book about outer space? Read to learn how Earth fits into the ever-changing solar system.

Earth is part of our solar system, which includes the sun and the objects that revolve around it. At the center of the solar system is the sun — a star, or ball of burning gases. The sun’s enormous mass, or the amount of matter it con-tains, creates a strong pull of gravity. This basic physical force keeps the Earth and the other objects revolving around the sun.

The PlanetsExcept for the sun, spheres called planets are the

largest objects in the solar system. At least eight planets exist, and each is in its own orbit around the sun. Mercury, Venus, Earth, and Mars are the inner planets, or those nearest the sun. Earth, the third planet from the sun, is about 93 million miles (about 150 million km) away. Farthest from the sun are the outer planets — Jupiter, Saturn, Uranus, and Neptune.

The planets vary in size. Jupiter is the largest. Earth ranks fifth in size, and Mercury is the smallest. All of the planets except Mercury and Venus have moons, smaller spheres or satellites that orbit them. Earth has 1 moon, and Saturn has at least 18 moons. Ceres and Pluto are dwarf

planets. Dwarf planets are small round bodies that orbit the sun, but have not cleared the area around their orbits of other orbiting bodies.

All of the planets are grouped into two types — terrestrial planets and gas giant planets. Mercury, Venus, Earth, and Mars are called terrestrial planets because they have solid, rocky crusts. Mercury and Venus are scalding hot, and Mars is a cold, barren desert. Only Earth has liquid water at the surface and can support varieties of life.

Farther from the sun are the gas giant planets — Jupiter, Saturn, Uranus, and Neptune. They are more gaseous and less dense than the terrestrial planets, even though they are larger in diameter. Each gas giant planet is like a minia-ture solar system, with orbiting moons and thin, encircling rings. Only Saturn’s rings, however, are easily seen from Earth by telescope.

Asteroids, Comets, and Meteoroids

Thousands of smaller objects—including aster-oids, comets, and meteoroids—revolve around the sun. Asteroids are small, irregularly shaped, planet-like objects. They are found mainly between Mars and Jupiter in the asteroid belt. A few asteroids follow paths that cross Earth’s orbit.

Comets, made of icy dust particles and frozen gases, look like bright balls with long, feathery tails. Their orbits are inclined at every possible angle to Earth’s orbit. They may approach from any direction.

The Solar System 1. Location Which four planets are closest to the sun?

2. Regions What physical characteristics do the inner planets share? The outer planets?

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Water, Land, and Air

32 Unit 1

Meteoroids are pieces of space debris — chunks of rock and iron. When they occasionally enter Earth’s atmosphere, friction usually burns them up before they reach the Earth’s surface. Those that collide with Earth are called meteorites. Meteorite strikes, though rare, can significantly affect the landscape, leaving craters and causing other devastation. In 1908 a huge area of forest in the remote Russian region of Siberia was flat-tened and burned by a “mysterious fireball.” Scientists theorize it was a meteorite or comet. A writer describes the effects:

‘‘The heat incinerated herds of reindeer and charred tens of thousands of evergreens across hundreds of square miles. For days, and for thousands of miles around, the sky remained bright with an eerie orange glow — as far away as western Europe people were able to read newspapers at night without a lamp.’’Richard Stone, “The Last Great Impact on Earth,”

Discover, September 1996

Movement Besides the planets, what other things revolve around the sun?

Getting to Know EarthMAIN MAIN IdeaIdea Earth’s surface is a complex mix of

landforms and water systems.

GEOGRAPHY AND YOU What do you know about Earth’s water, land, and air? Read to learn how these features support life on Earth.

The Earth is a rounded object wider around the center than from top to bottom. Earth has a larger diameter at the Equator — about 7,930 miles (12,760 km) — than from Pole to Pole, but the dif-ference is less than 1 percent. With a circumference of about 24,900 miles (40,060 km), Earth is the largest of the inner planets.

Water, Land, and AirThe surface of the Earth is made up of water

and land. About 70 percent of our planet’s sur-face is water. Oceans, lakes, rivers, and other bodies of water make up a part of the Earth called the hydrosphere.

About 30 percent of the Earth’s surface is land, including continents and islands. Land makes up

a part of the Earth called the lithosphere, the Earth’s crust. The lithosphere also includes the ocean basins, or the land beneath the oceans.

The air we breathe is part of Earth’s atmosphere, a layer of gases extending above the planet’s surface. The atmosphere is composed of 78 percent nitrogen, 21 percent oxygen, and small amounts of argon and other gases.

All people, animals, and plants live on or close to the Earth’s surface or in the atmosphere. The part of the Earth that supports life is the biosphere. Life outside the biosphere, such as on a space station orbiting Earth, exists only with the assis-tance of mechanical life-support systems.

The atmosphere, lithosphere, and hydrosphere form the biosphere, the part of Earth where life exists.

1. Place What are Earth’s water systems called?

2. Human-Environment Interaction How does human activity impact the biosphere?

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SECTION 1 REVIEWVocabulary 1. Explain the significance of: hydrosphere, lithosphere, atmo-

sphere, biosphere, continental shelf.

Main Ideas 2. List examples of Earth’s landforms and water systems. How do

these features help support life on our planet? 3. How are terrestrial planets and gas giant planets similar? How

are they different? 4. Use a diagram like the one below to describe Earth’s place in

the larger physical system that includes other planets, moons, and stars.

Critical Thinking 5. Think about Earth’s surface and list conditions that

must be present in a space station in order to support life. 6. Drawing Conclusions NASA has recently launched space

probes to explore Mars. Why might Mars have been chosen for these explorations?

7. Analyzing Visuals Study the diagram of the solar system on page 31. How is the size of a planet’s orbit influenced by its distance from the sun?

Writing About Geography 8. Descriptive Writing Consider the ratio of water and land on

Earth. Describe how Earth’s landforms and bodies of water would be different if the proportions were reversed.

LandformsLandforms are the natural features of the

Earth’s surface. So are bodies of water. The dia-gram on pages xxvi-xxvii shows many of the Earth’s landforms, which have a particular shape or elevation. Landforms often contain rivers, lakes, and streams.

Underwater landforms are as diverse as those found on dry land. In some places the ocean floor is a flat plain. Other parts feature moun-tain ranges, cliffs, valleys, and deep trenches.

Seen from space, Earth’s most visible land-forms are the seven large landmasses called con-tinents. Australia and Antarctica stand alone, while the others are joined in some way. Europe and Asia are parts of one landmass called Eurasia. A narrow strip of land called the Isthmus of Panama links North America and South America. At the Sinai Peninsula, the human-made Suez Canal separates Africa and Asia.

The continental shelf is an underwater extension of the coastal plain. Continental shelves slope out from land for as much as 800 miles (1,287 km) and descend gradually to a depth of about 660 feet (200 m), where a sharp drop marks the beginning of the continental slope. This area drops more sharply to the ocean floor.

Earth’s Heights and DepthsGreat contrasts exist in the heights and depths of

the Earth’s surface. The highest point on Earth is in South Asia at the top of Mount Everest, which is 29,035 feet (8,850 m) above sea level. The low-est dry land point, at 1,349 feet (411 m) below sea level, is the shore of the Dead Sea in Southwest Asia. Earth’s deepest known depression lies under the Pacific Ocean southwest of Guam in the Mariana Trench, a narrow, underwater canyon about 35,827 feet (10,920 m) deep.

Human-Environment InteractionHow does the biosphere support life on Earth?

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Planets

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Solar System

Chapter 2 33

SECTION

Guide to ReadingSection PreviewInternal and external forces shape the Earth‘s surface.

Content Vocabulary

Academic Vocabulary• releasing (p. 35)• constantly (p. 35)• tension (p. 38)

Places to Locate• Himalaya (p. 37)• San Andreas Fault (p. 37)• Ko-be (p. 38)• San Francisco (p. 38)• Ring of Fire (p. 38)• Greenland (p. 40)• Antarctica (p. 40)

Reading StrategyTaking Notes As you read about the forces that change the Earth, use the major headings of the sec-tion to create an outline similar to the one below.

Forces of Change

I. Earth’s Structure A. B. II. Internal Forces of Change A. B.

2 Forces of ChangeThe center of Earth is filled with intense heat and pressure. These natural forces drive numerous changes such as volcanoes and earthquakes that renew and enrich Earth’s surface. These physical processes can also disrupt, and often destroy, human life. As a result, scientists are working to learn how to predict them.

Voices Around the World“[S]cientists are doing everything they can to solve the mysteries of earthquakes. They break rocks in laboratories, studying how stone behaves under stress. They hike through ghost forests where dead trees tell of long-ago tsunamis. They make

maps of precarious, balanced rocks to see where the ground has shaken in the past, and how hard. They dig trenches across faults, search-

ing for the active trace. They have wired up fault zones with so many sensors it’s as though

the Earth is a patient in intensive care.”

— Joel Achenbach, “The Next Big One,” National Geographic,

April 2006

• core (p. 35)• mantle (p. 35)• crust (p. 35)• continental

drift (p. 35)• plate tectonics

(p. 35)• magma (p. 35)• subduction

(p. 37)• accretion (p. 37)

• spreading (p. 37)

• fold (p. 37)• fault (p. 37)• faulting (p. 38)• weathering

(p. 39)• erosion (p. 39)• glacier (p. 39)• moraine (p. 39)

34 Unit 1

A geologist studying earthquake activity

Reed Saxon/AP Images

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Chapter 2 35

Inside the EarthEarth’s StructureMAIN MAIN IdeaIdea The Earth’s internal and external

structure, including the tectonic plates, is responsi-ble for the creation of the continents, oceans, and mountain ranges.

GEOGRAPHY AND YOU Are there mountains near where you live? Read to learn how Earth’s moun-tains are formed.

For hundreds of millions of years, the surface of the Earth has been in slow but constant motion. Some forces that change the Earth, such as wind and water, occur on the surface. Others, such as volcanic eruptions and earthquakes, originate deep in the Earth’s interior.

A Layered PlanetThe diagram at the right shows that Earth is

composed of three main layers — the core, the mantle, and the crust. At the very center of the planet is a super-hot but solid inner core. Scientists believe that the inner core is made up of iron and nickel under enormous pressure. Surrounding the inner core is a band of melted iron and nickel called the liquid outer core.

Next to the outer core is a thick layer of hot, dense rock called the mantle. The mantle con-sists of silicon, aluminum, iron, magnesium, oxygen, and other elements. This mixture con-tinually rises, cools, sinks, warms up, and rises again, releasing 80 percent of the heat generated from the Earth’s interior.

The outer layer is the crust, a rocky shell form-ing the Earth’s surface. This relatively thin layer of rock ranges from about 2 miles (3.2 km) thick under oceans to about 75 miles (121 km) thick under mountains. The crust is broken into more than a dozen great slabs of rock called plates that rest — or more accurately, float — on a par-tially melted layer in the upper portion of the mantle. The plates carry the Earth’s oceans and continents.

Plate MovementIf you had seen the Earth from space 500 mil-

lion years ago, the planet probably would not have looked at all like it does today. Many sci-entists believe that most of the landmasses form-ing our present-day continents were once part of

one gigantic supercontinent called Pangaea(pan•JEE•uh). The maps on the next page show that over millions of years, this supercontinent has broken apart into smaller continents. These continents in turn have drifted and, in some places, recombined. The theory that the conti-nents were once joined and then slowly drifted apart is called continental drift.

The term plate tectonics refers to all of the physical processes that create many of the Earth’s physical features. Many scientists theorize that plates moving slowly around the globe have pro-duced Earth’s largest features — not only conti-nents, but also oceans and mountain ranges. Most of the time, plate movement is so gradual — only about 1 inch (2 to 3 cm) a year — that it cannot be felt. As they move, the plates may crash into each other, pull apart, or grind and slide past each other. Whatever their actions, plates are constantlychanging the face of the planet. They push up mountains, create volcanoes, and produce earth-quakes. When the plates spread apart, magma, or molten rock, is pushed up from the mantle, and ridges are formed. When plates bump together, one may slide under another, forming a trench.

1. Location Which of Earth’s layers is between the crust and the outer core?

2. Location How much does the temperature change from the inner core to the outer core? From the outer core to the mantle?

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36 Unit 1

Internal Forces of ChangeMAIN MAIN IdeaIdea Plate tectonics is responsible for fold-

ing, lifting, bending, and breaking parts of the Earth’s surface.

GEOGRAPHY AND YOU Have you seen news cover-age about earthquakes or volcanoes in different parts of the world? Read to learn how the internal forces of plate tectonics can cause such natural disasters.

Earth’s surface has changed greatly over time. Scientists believe that some of these changes come from forces associated with plate tecton-ics. One of these forces relates to the movement of magma within the Earth. Others involve movements that can fold, lift, bend, or break the solid rock at the Earth’s crust.

‘‘[W]e have a highly successful theory, called platetectonics, that explains . . . why continents drift,mountains rise, and volcanoes line the Pacific Rim. Plate tectonics may be one of the signature triumphs of the human mind. . . . ’’Joel Achenbach, “The Next Big One,”

National Geographic, April 2006

Many scientists estimate that plate tectonics has been shaping the Earth’s surface for 2.5 to 4 billion years. According to some scientists, plate tectonics will have sculpted a whole new look for our planet millions of years from now.

Scientists, however, have not yet determined exactly what causes plate tectonics. They theorize that heat rising from the Earth’s core may create slow-moving currents within the mantle. Over millions of years, these currents of molten rock may shift the plates around, but the movements are extremely slow and difficult to detect.

Movement What is the theory of continental drift?

Continental Drift

1. Regions How does the first map of Pangaea compare to the last map? The map of plate movement?

2. Movement Which plates are moving toward each other? Away from each other?

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with steep sides and sharp peaks, and piles up the resulting debris in trenches. This buildup can cause continents to grow outward. Most scientists believe that much of western North America expanded outward into the Pacific Ocean over more than 200 million years as a result of accretion.

New land can also form when two sea plates converge. In this process, one plate moves under the other, often forming an island chain at the boundary. Sea plates also can pull apart in a pro-cess known as spreading. The resulting rift, or deep crack, allows magma from within the Earth to well up between the plates. The magma hardens to build undersea volcanic mountains or ridges and some islands. This spreading activity occurs down the middle of the Atlantic Ocean’s floor, pushing Europe and North America away from each other.

Folds and FaultsMoving plates sometimes squeeze the Earth’s

surface until it buckles. This activity forms folds,or bends, in layers of rock. In other cases, plates may grind or slide past each other, creating cracks in the Earth’s crust called faults. One famous fault is the San Andreas Fault in California.

Colliding and Spreading PlatesMountains are formed in areas where giant con-

tinental plates collide. For example, the Himalayamountain ranges in South Asia were thrust upward when the Indian landmass drifted against Eurasia. Himalayan peaks are getting higher as the Indian landmass continues to move northward.

Mountains are also created when a sea plate collides with a continental plate. The diagram on page 38 shows how in a process called subduction (suhb•DUHK•shuhn) the heavier sea plate dives beneath the lighter continental plate. Plunging into the Earth’s interior, the sea plate becomes molten material. Then, as magma, it bursts through the crust to form volcanic mountains. The Andes, a mountain system in South America, was formed over millions of years as a result of subduction. The Nazca Plate slid beneath the South American Plate.

In other cases where continental and sea plates meet, a different process, known as accretion, occurs. During accretion (uh•KREE•shuhn), pieces of the Earth’s crust come together slowly as the sea plate slides under the continental plate. This move-ment levels off seamounts, underwater mountains

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Plate Movement

38 Unit 1

that cradle the Pacific meet the plates that hold the continents surrounding the Pacific. North America, South America, Asia, and Australia are affected by their location on the Ring of Fire.

Volcanic EruptionsVolcanoes are mountains formed by lava or by

magma that breaks through the Earth’s crust. Volcanoes often rise along plate boundaries where one plate plunges beneath another, as along the Ring of Fire. In such a process, the rocky plate melts as it dives downward into the hot mantle. If the molten rock is too thick, its flow is blocked and pressure builds. A cloud of ash and gas may then spew forth, creating a fun-nel through which the red-hot magma rushes to the surface. There the lava flow may eventually form a large volcanic cone topped by a crater, a bowl-shaped depression at a volcano’s mouth.

Volcanoes also arise in areas away from plate boundaries. Some areas deep in the Earth are hot-ter than others, and magma often blasts through the surface as volcanoes. As a moving plate passes over these hot spots, molten rock flowing out of the Earth may create volcanic island chains, such as the Hawaiian Islands. At some hot spots, mol-ten rock may also heat underground water, caus-ing hot springs or geysers like Old Faithful in Yellowstone National Park.

Location Where is the Ring of Fire?

The process of faulting occurs when the folded land cannot be bent any further. Then the Earth’s crust cracks and breaks into huge blocks. The blocks move along the faults in different direc-tions, grinding against each other. The resulting tension may release a series of small jumps, felt as minor tremors on the Earth’s surface.

EarthquakesSudden, violent movements of tectonic plates

along a fault line are known as earthquakes. These shaking activities dramatically change the surface of the land and the floor of the ocean. During a severe earthquake in Alaska in 1964, a portion of the ground lurched upward 38 feet (11.6 m).

Earthquakes often occur where plates meet. Tension builds up along fault lines as the plates stick. The strain eventually becomes so intense that the rocks suddenly snap and shift. This movement releases stored-up energy along the fault. The ground then trembles and shakes as shock waves surge through it moving away from the area where the rocks first snapped apart.

In recent years disastrous earthquakes have occurred in Kobe, Japan, and in Los Angeles and San Francisco. These cities are located along the Ring of Fire, one of the most earthquake-prone areas on the planet. The Ring of Fire is a zone of earthquake and volcanic activity around the perimeter of the Pacific Ocean. Here the plates

Forces of Change

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Chapter 2 39

Wind ErosionWind erosion involves the movement of dust,

sand, and soil from one place to another. Plants help protect the land from wind erosion; how-ever, in dry places where people have cut down trees and plants, winds pick up large amounts of soil and blow it away. Wind erosion can provide some benefits. The dust carried by wind often forms large deposits of mineral-rich soil.

Glacial ErosionAnother cause of erosion is glaciers, or large

bodies of ice that move across the Earth’s surface. Glaciers form over time as layers of snow press together and turn to ice. Their great weight causes them to move slowly downhill or spread out-ward. As they move, glaciers pick up rocks and soil in their paths, changing the landscape. They can destroy forests, carve out valleys, alter the courses of rivers, and wear down mountaintops.

When glaciers melt and recede, in some places they leave behind large piles of rocks and debris called moraines. Some moraines form long ridges of land, while others form dams that hold water back and create glacial lakes.

External Forces of ChangeMAIN MAIN IdeaIdea External forces such as weathering

and erosion also shape the surface of the Earth.

GEOGRAPHY AND YOU Have you ever seen soil washed over the road after a heavy rain? Read to learn how wind and rain can shape Earth’s surface.

External forces, such as wind and water, also change the Earth’s surface. Wind and water move-ments involve two processes. Weathering breaks down rocks and erosion wears away the Earth’s surface by wind, glaciers, and moving water.

WeatheringThe Earth is changed by two basic kinds of

weathering. Physical weathering occurs when large masses of rock are physically broken down into smaller pieces. For example, water seeps into the cracks in a rock and freezes, then expands and causes the rock to split. Chemical weathering changes the chemical makeup of rocks. For example, rain water that contains car-bon dioxide from the air easily dissolves certain rocks, such as limestone. Many of the world’s caves have been and continue to be formed by this process. 1. Place How does accretion create deep trenches?

2. Human-Environment Interaction How have human settlements been affected by the process of faulting?

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Vocabulary 1. Explain the significance of: core, mantle, crust, continental drift,

plate tectonics, magma, subduction, accretion, spreading, fold, fault, faulting, weathering, erosion, glacier, moraine.

Main Ideas 2. How does the internal structure of the Earth influence the

creation of continents, oceans, and mountain ranges? 3. Describe the two kinds of weathering and the three kinds of

erosion that shape the surface of the Earth. 4. Use a chart like the one below to explain how plate tectonics

is responsible for folding, lifting, bending, and breaking parts of Earth’s surface.

Critical Thinking 5. Based on your understanding of plate tectonics,

what changes would you predict to the Earth’s appearance millions of years from now?

6. Drawing Conclusions In what ways can erosion be both beneficial and harmful to agricultural communities?

7. Analyzing Visuals Study the map of plate movement on page 37. Which plates are responsible for the earthquakes that have occurred in California?

Writing About Geography 8. Descriptive Writing Review how internal forces shape the

surface of the Earth. Now imagine that the mantle ceased to circulate molten rock. Write a description of how land forma-tion on the surface of the Earth would be different.

SECTION 2 REVIEW

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40 Unit 1

There are two types of glaciers . Sheet glaciers are flat, broad sheets of ice. Today sheet glaciers cover most of Greenland and all of Antarctica.They advance a few feet each winter and recede in the summer. Large blocks of ice often break off from the coastal edges of sheet glaciers to become icebergs floating in the ocean. Mountain glaciers, located in high mountain valleys where the climate is cold, gouge out round, U-shaped valleys as they move downhill.

Water ErosionWater erosion begins when springwater and

rainwater flow downhill in streams, cutting into the land, and wearing away the soil and rock. The resulting sediment grinds away the surface of rocks along the stream’s path. Over time, the eroding action of water forms first a gully and then a V-shaped valley. Sometimes valleys are eroded even further to form canyons. The Grand Canyon is an example of the eroding power of water.

Oceans also play an important role in water erosion. Pounding waves continually erode coastal cliffs, wear rocks into sandy beaches, and move sand away to other coastal areas.

Regions How have many of the world’s caves been formed?

Forces of Change

Process How It Works Example

Subduction

Fast-moving water, the most significant cause

of erosion, helped create the Delicate Arch in Arches National Park near Moab, Utah.

Movement How does water erosion form valleys and canyons?

ML Sinibaldi/CORBIS

SECTION 3 Earth’s WaterA submarine crew investigating the Arctic Ocean can still experience the thrill of exploring uncharted terri-tory — one of Earth’s last frontiers. Although humans live mostly on land, water is important to our lives, and all living things need water to survive.

Voices Around the World“In a world that’s been almost completely mapped, it’s easy to forget why car-tographers used to put monsters in the blank spots. Today we got a reminder. The submarine captain had warned us that we were in uncharted waters. . . . Yet the first days of our cruise through this ice-covered ocean, Earth’s least explored frontier, were . . . smooth. . . . Even when we passed over a mile-high mountain that no one on the planet knew existed, the reaction was one of quiet enthusiasm — ‘Neat.’ ”

— Glenn Hodges, “The New Cold War,” National Geographic,

March 2000

Guide to ReadingSection PreviewEarth’s water cycle keeps the amount of water on our planet, both freshwater and salt water, fairly constant.

Content Vocabulary• water cycle (p. 42)• evaporation (p. 42)• condensation (p. 42)• precipitation (p. 42)• desalination (p. 43)• groundwater (p. 44)• aquifer (p. 44)

Academic Vocabulary• area (p. 43)• focus (p. 43)• source (p. 44)

Places to Locate• Pacific Ocean (p. 43)• Atlantic Ocean (p. 43)• Indian Ocean (p. 43)• Arctic Ocean (p. 43)• Mediterranean Sea (p. 43)• Gulf of Mexico (p. 43)

Reading StrategyOrganizing As you read, complete a graphic organizer similar to the one below by listing the processes that contribute to the water cycle.

Water Cycle

A pilot aboard a submarine research vessel

Chapter 2 41

Sandra Teddy/Getty Images

42 Unit 1

The Water CycleMAIN MAIN IdeaIdea The amount of water on Earth remains

fairly constant and moves in the water cycle.

GEOGRAPHY AND YOU Have you ever experi-enced a day when the air just feels heavy and wet? Read to learn what causes such conditions and how it is related to the water cycle.

As you recall, oceans, lakes, rivers, and other bodies of water make up the Earth’s hydro-sphere. Almost all of the hydrosphere is salt water found in the oceans, seas, and a few large saltwater lakes. The remainder is freshwater found in lakes, rivers, and groundwater.

The total amount of water on Earth does not change, but the Earth’s water is constantly mov-ing — from the oceans to the air to the land and finally back to the oceans. The water cycle is the name given to this regular movement of water. The diagram below shows how the water cycle works.

The sun drives the cycle by evaporating water from the surfaces of oceans, lakes, and streams. Evaporation is the changing of liquid water into vapor, or gas. The sun’s energy causes evaporation.

Water vapor rising from the oceans, other bod-ies of water, and plants is gathered in the air. The amount of water vapor the air holds depends on its temperature. Warm, less dense air holds more water vapor than does cool air.

When warm air cools, it cannot retain all of its water vapor, so the excess water vapor changes into liquid water — a process called condensation.Tiny droplets of water come together to form clouds. When clouds gather more water than they can hold, they release moisture, which falls to the Earth as precipitation — rain, snow, or sleet, depending on the air temperature and wind con-ditions. This precipitation sinks into the ground and collects in streams and lakes to return to the oceans. Soon most of it evaporates, and the cycle begins again.

The amount of water that evaporates is approximately the same amount that falls back to the Earth. This amount varies little from year to year. Thus, the total volume of water in the water cycle is more or less constant.

Regions What drives the Earth’s water cycle?

The Water Cycle

1. Movement In which part of the water cycle does liquid become vapor?

2. Human-Environment Interaction How would life on Earth be different if there were no precipitation?

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Chapter 2 43

Bodies of Salt WaterMAIN MAIN IdeaIdea Salt water covers much of the Earth’s

surface.

GEOGRAPHY AND YOU Have you ever been swim-ming in the ocean? Read to learn about Earth’s bodies of salt water.

Seen from space, the Earth’s oceans and seas are more prominent than its landmasses. About 70 percent of the Earth’s surface is water, but almost all of it is salt water. Freshwater makes up only a small percentage of Earth’s water.

OceansAbout 97 percent of the Earth’s water consists

of a huge, continuous body of water that circles the planet. Geographers divide this enormous expanse into four oceans: the Pacific, the Atlantic, the Indian, and the Arctic. The Pacific, the larg-est of the oceans, covers more area than all the Earth’s land combined. The Pacific Ocean is also deep enough in some places to cover Mount Everest, the world’s highest mountain, with more than 1 mile (1.6 km) to spare. The immense size of Earth’s oceans continues to inspire awe and fascination in humans.

Seas, gulfs, and bays are bodies of salt water smaller than oceans. These bodies of water are often partially enclosed by land. As one of the world’s largest seas, the Mediter ranean Sea is almost entirely encircled by southern Europe, northern Africa, and southwestern Asia. The Gulf of Mexico is nearly encircled by the coasts of the United States and Mexico. Scientists have identified 66 separate seas, gulfs, and bays and many smaller divisions.

Salt Water to Fresh waterAlthough 97 percent of the world’s water is

found in oceans, the water is too salty for drink-ing, farming, or manufacturing. Today efforts focus on ways to meet the world’s increasing need for freshwater, such as turning ocean water into freshwater by removing the salt in a pro-cess known as desalination. The diagram above shows one way that salt water can be turned into freshwater.

Because desalination is expensive, only a small amount of freshwater is obtained this way. Even so, certain countries in Southwest Asia and North Africa use desalination because other freshwater sources are scarce. Many of these countries also have the energy sup plies to run the desalination plants cheaply.

‘‘That the planet’s fresh water is consumed profli-gately is beyond doubt, particularly in agriculture, which accounts for 70 percent of all water use. Getting more out of each drop of water is impera-tive, for as the world’s population increases and the demand for food soars, unchecked irrigation poses a serious threat to rivers, wetlands, and lakes.’’ Fen Montaigne, “Water Pressure,”

National Geographic, September 2002

Regions What are bodies of salt water smaller than oceans?

Geography ONLINE

Student Web Activity Visit the World Geography and Cultures Web site at glencoe.com and click on Student Web Activities—Chapter 2 for an activity about the Earth’s oceans.

Vocabulary 1. Explain the significance of: water cycle, evaporation, conden-

sation, precipitation, desalination, groundwater, aquifer.

Main Ideas 2. How does the water cycle function to maintain a fairly con-

stant amount of water on the Earth? 3. Describe the bodies of salt water that cover much of the

Earth’s surface. 4. Use a web diagram like the one below to identify the bodies

of freshwater that are necessary to sustain life on Earth.

Critical Thinking 5. Use your knowledge of the water cycle to explain

how droughts might occur. 6. Making Inferences Why might salt water someday provide

water for drinking, farming, and manufacturing? 7. Identifying Cause and Effect When drinking water is con-

taminated by hazardous substances, how is it released on land or into rivers and lakes?

8. Analyzing Visuals Look at the diagram of the water cycle on page 42. What source of water supplies wells and springs?

Writing About Geography 9. Expository Writing Many large urban areas developed in

river basins. Write a description of how a river or rivers con-tributed to the development of your community.

SECTION 3 REVIEW

Bodies of FreshwaterMAIN MAIN IdeaIdea Although there is a small amount of

freshwater on Earth, it is necessary to sustain life.

GEOGRAPHY AND YOU Does your family or com-munity get its water from a well? Read to learn about groundwater, one of Earth’s freshwater sources.

Only about 3 percent of the Earth’s total water supply is freshwater, and most is not available for human consumption. More than 2 percent is fro-zen in glaciers and ice caps. Another 0.5 percent is found beneath the Earth’s surface. Lakes, streams, and rivers contain less than 1 percent of the Earth’s water.

Lakes, Streams, and RiversA lake is a body of water completely sur-

rounded by land. Most lakes contain freshwater, although some, such as Southwest Asia’s Dead Sea and Utah’s Great Salt Lake, are saltwater remnants of ancient seas. Most lakes are found where glacial movement has cut deep valleys and built up dams of soil and rock that held back melting ice water. North America has thou-sands of glacial lakes.

Flowing water forms streams and rivers. Melt-water, an overflowing lake, or a spring may be the source, or beginning, of a stream. Streams may combine to form a river, a larger stream of higher volume that follows a channel along a par-ticular course. When rivers join, the major river systems that result may flow for thousands of miles. Rain, runoff, and water from tributaries or branches swell rivers as they flow toward a lake, gulf, sea, or ocean. The place where the river emp-ties into another body of water is its mouth.

GroundwaterGroundwater, freshwater that lies beneath the

Earth’s surface, comes from rain and melted snow that filter through the soil and from water that seeps into the ground from lakes and rivers. Wells and springs tap into groundwater and are important sources of freshwater for people in many rural areas and in some cities. An under-ground porous rock layer often saturated by very slow flows of water is called an aquifer(A•kwuh•fuhr). Aquifers and groundwater are important sources of freshwater.

Place What are the sources of Earth’s freshwater?

44 Unit 1

Geography ONLINE

Study Central™ To review this section, go to glencoe.com and click on Study Central.

Earth’s Freshwater Features

• Earth is the third planet from the sun in our solar system. It is one of 4 planets with a solid, rocky crust, and the only planet with liquid water on its surface.

• Earth’s proximity to the sun allows for liquid water on the surface needed to sustain life.

• Earth also has an atmosphere that protects life from the harmful effects of the sun and keeps the Earth at a temperature suitable for a variety of life forms.

PLANET EARTH

• The Earth is shaped by internal and external forces. • Inside the Earth is a superheated, solid inner core. A liquid

outer core is covered by the hot rock of the mantle.• The Earth’s crust, made up of more than a dozen slabs, rests on

a melted layer of mantle. These slabs move around the globe, creating physical features.

• Wind and water erosion shape the surface of the Earth.

FORCES OF CHANGE

• The water cycle keeps Earth’s water constantly moving—water evaporates from oceans, lakes and streams. Then it cools, becomes condensation, and falls to Earth as precipitation.

• Most of Earth’s water is found in the oceans. The rest is frozen in glaciers, found underground, or in lakes, rivers and streams.

• Because most of Earth’s water is salty ocean water, people have found a way to remove the salt from water. Desalination is expensive, but useful in places where freshwater is scarce.

EARTH’S WATER

The San Andreas Fault, California

Earth from Space

Earth’s Available Water

THE W

ORLD

Chapter 2 45

CHAPTER VISUAL SUMMARY Study anywhere, anytime by downloading quizzes and flashcards to your PDA from glencoe.com.2

(bkgd)Robert Laberge/Getty Images, (tr)NASA/CORBIS, (cl)Tom Bean/CORBIS

GO ON

STANDARDIZED TEST PRACTICE

TEST TAKING TIP

46 Unit 1

CHAPTER 2

Reviewing VocabularyDirections: Choose the word or words that best complete the sentence.

1. The is the Earth’s land or crust.

A hydrosphere

B atmosphere

C biosphere

D lithosphere

2. According to the theory of , the Earth’s continents were once joined in one large landmass and then separated.

A evolution

B motion

C continental drift

D relativity

3. In the process of , Earth’s surface is worn down by wind, glaciers, and water.

A folding

B plate tectonics

C erosion

D weathering

4. A saturated, porous, underground layer of rock through which water fl ows slowly is .

A a spring

B an aquifer

C a desalinization

D a pool

Reviewing Main IdeasDirections: Choose the best answers to the following questions.

Section 1 (pp. 30–33)

5. Gravity is the physical force that

A creates the asteroid belt.

B is due to the sun’s small mass and the lack of matter it contains.

C establishes and maintains the order of the planets.

D keeps the planets and other objects revolving around the sun.

Section 2 (pp. 34–40)

6. The force that does most of the work of wearing down Earth is .

A wind

B sand

C water

D heat

Section 3 (pp. 41–44)

7. The total amount of water in the hydrosphere generally .

A decreases

B increases

C stays the same

D decreases, then increases

Scan the entire test section before you begin. Answer the questions that are easiest for you fi rst.

TEST-TAKING TIP

Caribbean Sea

Gulf of Mexico

PA C I F I C O C E A N

Lambert AzimuthalEqual-Area projection

N

S

W E

Plate Movement

STOP

Chapter 2 47

ASSESSMENT

If you missed questions. . . 1 2 3 4 5 6 7 8 9 10 11 12 13Go to page. . . 32 35 39 44 31 40 42 32 37 39 40 32 39

Need Extra Help?

Geography ONLINE

For additional test practice, use Self-Check Quizzes—Chapter 2 on glencoe.com.

Critical ThinkingDirections: Choose the best answers to the following questions.

8. How are the hydrosphere, the lithosphere, and the atmo-sphere related?

A All are circular regions.

B They are all elements that make up the biosphere.

C People have no control over the hydrosphere, the litho-sphere, or the atmosphere.

D All three revolve around the sun.

Base your answer to question 9 on the map and on your knowledge of Chapter 2.

9. Which plates collided to help form the Andes?

A South American and Nazca

B Caribbean and Cocos

C North American and Nazca

D South American and Caribbean

Document-Based QuestionsDirections: Analyze the document and answer the short-answer ques-tions that follow the document.

Human actions can make natural disasters worse. When a strong earthquake struck northern Pakistan in October 2005, the dam-age was made worse by human effects on the environment.

This month’s massive earthquake did not destroy Mohammad Shafi Mir’s house and bury his mother, but what followed seconds later did: a torrent of boulders thundering down a mountainside. . . .

The landslides that tumbled across the zone of the Oct. 8 earthquake dramatized not only the power of nature but how humans have brought tragedy upon themselves through massive deforestation and other ecological assaults on the Himalayas.

In this once-remote region, loss of green cover from com-mercial logging, local cutting and overgrazing has weakened the land’s ability to retain water, which now rushes easily down mountainsides to set off slides that some call “ecologi-cal land mines.”

—”Earthquake Dramatizes Human Ecological Assault on the Himalayas,“ October 22, 2005, Associated Press Archive

10. What human activity has made the effect of a strong earth-quake worse?

11. What does the writer mean by “ecological land mines”?

Extended Response12. Recall what you have learned about the lithosphere, hydro-

sphere, and atmosphere. How do they affect human life?

13. What are the different types of erosion? How does each affect the land?